The invention relates generally to optics and communications, and more specifically to optical fiber based networks, techniques for restoration of network services in the event of a failed fiber link (e.g., a break in a fiber or a failure of an active element such as a fiber amplifier) and the use of optical switching to effect such restoration.
Photonic transmission, amplification, and switching techniques provide flexible means of provisioning, configuring, and managing the modern high capacity telecommunication networks. The physical layer in the network, which includes the transmission equipment and the fiber layer used for signal transport, is required to be capable of reconfiguration of facilities in order to support dynamic routing of traffic. While slow reconfiguration of the order of minutes or more may be sufficient for rearranging traffic capacity in response to change in demand patterns across the network, rapid reconfiguration (perhaps 50 ms or less) is required for restoring services in the case of transmission equipment or fiber cable facility failures. Fast restoration is also critical to prevent escalation of the effects of a single point of failure where the affected services (voice and data) attempt to reconnect immediately following the disruption of services and may lead to overloading of facilities adjacent or connected to the point of original failure.
In addition to the critical need for fast restoration, the capacity that needs to be re-routed has increased rapidly with the continuing increase in data rates for optical transmission and the introduction of multi-wavelength channels on a single fiber. For example, the rapid growth in traffic capacities required for long haul telecommunications networks has accelerated the introduction of new technologies for transmission and multiplexing. Transmission links up to bit rates of 10 Gbps (OC-192) are in commercial service and new developments in multi-wavelength component technologies are resulting in increased commercial availability of 4-, 8-, 16-, 32-, and 40-channel WDM (wavelength division multiplex) links (at 2.5 Gbps per wavelength or more).
Transmission of such high data rates over single fibers also results in making the network more vulnerable to failures of larger magnitude. For example, a single fiber link failure can disrupt approximately 130,000 voice channels (DS0) when the fiber link is operating at 10 Gbps on a single-wavelength or at 2.5 Gbps on each of four wavelengths. Consequently, redundant facilities provisioned for dynamic restoration of service also need to provide a similar magnitude of capacity on the links used as backup or spare links for ensuring network survivability.
Therefore, routing techniques used for network restoration must provide solutions that are compatible with the twofold requirement of fast switching and high capacity.
International and North American standard bodies have defined various Synchronous Optical Network (SONET) configurations for operation of lightwave networks. "Self-healing ring" configurations allow for rapid restoration of services in the event of a failure of fiber transmission media. In a four-fiber self-healing ring network, each node is connected to its adjacent nodes through two pairs of fibers (carrying signals in opposite directions). One fiber in each such pair is called the "working" fiber; the other fiber is termed the "protection" fiber and may be used when the working fiber facility fails. Each node includes add-drop multiplexer (ADM) terminal equipment that originates and terminates signals traversing the various links in the ring.
When a failure of any working fiber link between any two nodes occurs, the ADM terminal equipment on either side of the failure carries out the required re-routing of signals over protection fibers. Such re-routing of signals to restore all services is referred to as "restoration" of services. If an outgoing working fiber link fails, but the corresponding protection fiber link is intact, the signals intended for the failed working fiber will be diverted to the intact corresponding protection fiber in what is referred to as span switching. In this context, reference to the corresponding protection fiber means the protection fiber coupled between the same two nodes and for use in the same direction (to or from the other node).
If the working and protection links fail, the signals intended for the failed working fiber will be directed to the outgoing protection fiber in the other direction around the ring, being passed from one node to the next, in what is referred to as ring switching.
However, some of these restoration schemes (ring switching) break down in what will be referred to as heterogeneous networks. A heterogeneous ring network is one where different links differ in some material respect such as signal-carrying capacity (bandwidth), number of wavelength channels, modulation scheme, format, or protocol. For example, certain high-traffic links may have been upgraded to provide increased bandwidth, by increasing the bit rate of signals on a given wavelength channel, by providing additional WDM terminal equipment to support additional wavelength channels, or both.
Thus, for a variety of reasons, the network may have a link, with terminal equipment at each end, where the signals on that link are alien or unsupported on one or more other links. Since at least some link in the opposite direction will not support the signals that normally travel on the failed link, ring switching is not possible. A particular type of heterogeneous network, namely one containing single- and multi-wavelength lightwave communication links, is sometimes referred to as a hybrid network.